Our primary purpose is to test the hypothesis that Black-white differences in the age-specific incidence of breast cancer can be attributed chiefly to racial differences in social class distribution: among women over age 40, the incidence is higher in whites as compared to Blacks, while in women under age 40, the incidence is higher in Blacks as compared to whites. Researchers have explained by former by invoking factors associated with the overall higher social class standing of whites, yet neither this hypothesis nor other currently known risk factors underlying the racial cross-over in rates, it is necessary to determine if "race" is acting chiefly as a social, rather than biological, variable. If this study can demonstrate that the age-specific incidence curves of Black and white women vary by social class and that the curves are similar for Black and White women of comparable social class standing, then the results would suggest that future etiologic studies and research on Black/white differences in breast cancer occurrence should address the ways in which both lower and higher social class standing mediate breast cancer etiology. To determine the age-specific breast cancer rates by race and social class, we would identify all cases diagnosed between January and December 1980 in the San Francisco-Oakland SMSA SEER registry (1524 whites, 137 Blacks) and assign them a set of social class indicators based on their 1980 census tract and census block-group characteristics. Then, separately grouping Black and white cases by age-group and social class, we would divide the number of cases by the corresponding number of Black and white women of the same social class and age-group within the San Francisco- Oakland SMSA. A secondary goal of this study is to assess the validity or limitations of using census-derived social class data in conjunction with individual registry data to analyze social class trends in cancer morbidity and mortality. To do this, we would interview a representative population of 100 women (50 Black, 50 white), selected at random from Alameda County. We would then compare the concordance of both their past and present individual social class indicators with their 1980 census tract and census block-group data.